The present invention relates well pumping and more specifically oil well pumping within the oil exploration and collection sector. In particular, the present invention is directed to a well pumping unit wherein the pumping unit is driven by a linear motor.
Traditionally, most well pumping units use motors that produce rotary motion as power for the unit. These units require a reducing gear box and force transforming structure to transform the rotary motion into linear motion to operate the polished rod up and down and thereby operate the pump. These well pumping units require a large and complex body structure and consume large amounts of power.
Chinese ZL Patent No. 96217638.9 (CN 2284867Y filed Jul. 22, 1996) is directed to a well pumping unit driven by a linear motor. This technology adopts a linear motor that links to and drives the polished rod and rod pump, thus removing the gear box and force transforming structure required by units driven by rotary motion. However, the unit's linear motor complex secondary and longitudinal position limiting switches are fixed on the wellhead; positioning blocks are fixed on both ends of the complex primary and slide against the position-limiting post, and the wellhead unit and the position-limiting post are respectively fixed with buffer springs. The complex primary of the linear motor is hollow in structure through which the polished rod passes. The polished rod and its clamping mechanism are fixed on the complex primary of the linear motor. There are counter weight slideway wheels on the support. The clamping mechanism fixed to the complex primary is linked to the counter weight by way of a steel cable that passes over the slideway wheels.
The previous well pumping unit driven by a linear motor, wherein the complex secondary of the linear motor and the position-limiting post are fixed on the wellhead assembly, has some unavoidable drawbacks. First, motion of the linear motor causes vibration in the pumping unit and this is transferred directly to the wellhead unit. Since the complex secondary and the position-limiting post are fixed on the well, the motion of the complex primary makes the wellhead unit vibrate, which weakens the wellhead unit seals and thus causes oil and/or natural gas leakage. When natural gas leakage occurs and the linear motor is directly fixed to the wellhead unit, natural gas can spread directly into the linear motor, which presents a serious risk of explosion. Second, since the linear motor is fixed on the wellhead unit covering the opening part of the well, it makes maintenance of the wellhead unit and downhole components, such as changes of the packing and/or the packing set, difficult. Third, since the linear motor is fixed on the wellhead, and the wellhead unit is linked to the downhole casing pipe, vibration of the motor makes the casing pipe vibrate, which in turn causes the formation to vibrate and the casing pipe to deform. Fourth, since the linear motor is fixed on the wellhead, the wellhead unit cannot withstand large pressure. Thus, if the linear motor is fixed on the wellhead, additional design is required for the wellhead unit to withstand the large pressure, which requires more investment and cost associated with the oil recovery equipment.
It is therefore an object of the present invention to provide a well pumping unit driven by a linear motor that is removed from the wellhead unit, and thus provides an improved well pumping unit that overcomes the drawbacks associated with known pumping units. These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.